(1) Field of the Invention
The present invention relates to a powdery or a particulate oxygen-absorbing component which is excellent in oxygen-absorbing performance and which can be used in a wide humidity range of a dry state to a highly humid state, an oxygen absorbent package, and an oxygen-absorbing monolayered body and multilayered body, each containing this oxygen-absorbing component.
An oxygen-absorbing component of the present invention can be used directly or as a mixture of the same, an adsorbing component and the like in the form of sachets, the form of a resin which is kneaded with the oxygen-absorbing component, or the form of a package comprising a sheet-like or a film-like oxygen-absorbing monolayered body or multilayered body obtained by molding a resin containing the oxygen-absorbing component for the purpose of preventing the oxidation of various products such as foods, drugs, metallic products and electronic products which are liable to be affected by oxygen and to bring about a quality change.
(2) Description of the Related Art
For the purpose of preventing the oxidation of various products typified by foods, drugs, metallic products and electronic products which easily change the quality under the influence of oxygen, there have heretofore been used oxygen absorbents which can remove oxygen from packaging containers or packaging bags receiving these products. Most of these oxygen absorbents which had been developed in former days and are presently used, are in the form of sachets wherein the powdery or the particulate oxygen-absorbing component is contained in a packaging material (Japanese Patent Publication Nos. 50618/1981, 6846/1987, etc.). In addition, other forms such as a label type, a card type and a packing type have also been used in which a piece of a monolayered or a multilayered sheet containing a layer comprising a resin kneaded with the oxygen-absorbing component is put in a packaging material (Japanese Patent Application Laid-open Nos. 219430/1995, 137759/1995, etc.).
Furthermore, as another safe oxygen-absorbing body which can easily be handled, can be applied in a wide range and can prevent eating by mistake, a form such as a film or a sheet (the film and the sheet will hereinafter be referred to as "the film" together) has been contrived.
In order to form the oxygen-absorbing component into the film while its oxygen-absorbing function is maintained, it is an easy and convenient way to immobilize the oxygen-absorbing component by making a composite of the powdery or the particulate oxygen-absorbing component and a thermoplastic resin as a matrix component. Furthermore, if the monolayered film of this composite is directly used, the film contacts the content, which leads to the contamination of the content. To avoid this inconvenience, a multilayered structure in which both the sides of the oxygen-absorbing layer are covered with resin layers has been contrived. In addition, when either of both the resin layers can be made of a gas-barrier material, both of a gas-barrier function and an oxygen-absorbing function can be kept by the multilayered film alone. These techniques have been disclosed in Japanese Patent Publication Nos. 1824/1987, 2648/1988, etc.
Moreover, in order to increase the oxygen permeability of the oxygen-absorbing layer or the oxygen permeability of a resin layer other than the gas-barrier layer which covers the oxygen-absorbing layer (or both the resin layers in the case of no gas-barrier layer), there have been contrived a technique of making the oxygen-absorbing layer porous, a technique of making the resin layer covering the oxygen-absorbing layer porous, the use of a thin non-porous resin layer, the simultaneous use of a thin non-porous resin layer and a porous resin layer, etc. Here, as a method of making the layer porous, a resin including an iron powder as the oxygen-absorbing component and a filler hardly soluble in water is drawn. Some examples of these techniques have been disclosed in Japanese Patent Application Laid-Open Nos. 72851/1990, 162251/1993, 318675/1993, 234811/1997, etc.
Furthermore, in addition to a further positive implementation of the oxygen-absorbing function, a way of reducing the permeability of oxygen from the outside by substantially improving the gas-barrier properties of a packaging material has also been contrived. Concretely, in this way, there is used a resin layer comprising various resins containing a small amount of a metal catalyst, and the resin layer can be covered with another resin layer. Some examples of these techniques have been disclosed in Japanese PCT Patent Application Laid-Open No. 500846/1990, Japanese Patent Application Laid-Open Nos. 269044/1991, 97163/1993, 115776/1993, 305973/1993, 48474/1994, etc.
As described above, many proposals have been made on the packaging materials in which the resin layer of a monolayered body or the partial resin layers of a multilayered body absorb oxygen as a whole. In this case, however, there is a problem that, with the progress of the oxygen absorption, the whole absorption layer becomes oxidized and degraded, so that the strength of the packaging material inconveniently deteriorates.
As the oxygen-absorbing component, various metals or metallic compounds are presently used, and particularly, an iron powder is most often used. However, in order to cause the oxidation of a metallic powder such as this iron powder, water is necessary, and when a system which is an object of the oxygen-absorbing contains little water (hereinafter referred to as "the dry system"), the oxygen absorption does not occur or an oxygen absorption rate is extremely slow.
In this connection, as oxygen-absorbing components applicable to the dry system as well as a highly humid system, reaction systems including various organic compounds as materials to be oxidized have been developed. Examples of these oxygen-absorbing components include systems which contain carbon-carbon unsaturated bonds, i.e., low molecular systems (Japanese Patent Publication Nos. 28266/1985 and 60936/1987) and high molecular systems (Japanese Patent Application Laid-Open Nos. 29741/1992 and 115776/1993), and systems which scarcely contain the carbon-carbon unsaturated bonds, i.e., low molecular systems such as alcohols (Japanese Patent Application Laid-Open No. 8441/1991), and various high molecular systems such as polyolefins (Japanese Patent Application Laid-Open No. 187238/1992), ethylene-vinyl alcohol copolymers (Japanese Patent Publication No. 49354/1994), polyvinyl chlorides (Japanese Patent Application Laid-Open No. 269044/1991) and polyamides (Japanese PCT Patent Application Laid-Open No. 500846/1990). Every system mentioned above contains a metal or a metallic compound as a catalyst for accelerating the reaction with oxygen.
On the other hand, in the case where the oxygen-absorbing component for the dry system is practically used, this component is preferably a solid which can be easily handled. Furthermore, similarly to a usual solid reaction, the surface area of the solid is preferably increased to accelerate the oxidation reaction. From such a viewpoint, techniques of obtaining the powdery or the particulate solid have been developed, and their examples include a system in which a low molecular compound (a liquid) is supported on a carrier or solidified (Japanese Patent Publication Nos. 60936/1987 and 11056/1996) and a system in which lumps of a polymer (a solid) are ground (Japanese Patent Application Laid-Open No. 187238/1992). However, when the liquid is supported or solidified, there is a risk that a liquid elutes, and when the solid lumps are ground, this grinding is required to be done at a low temperature, because the above-mentioned polymer (a thermoplastic resin) is flexible in the vicinity of room temperature. In addition, there is a problem that, in a part of the polymer systems having a low glass transition temperature and containing the carbon-carbon unsaturated bonds, even after the grinding is once done at a low temperature, the powdery granules or the particles re-adhere to each other to become large lumps, when returned to room temperature.
As described above, the oxygen-absorbing component for the dry system is already known, but an easily handleable powdery or the particulate oxygen-absorbing component in which all constituents are in solid form has never existed so far.